1. Field of the Invention
The invention relates to an alpha-beta titanium-base alloy having an outstanding combination of tensile strength, including shear strength and ductility.
2. Description of the Prior Art
There have been numerous titanium alloys developed since the titanium industry started in earnest in the early 1950's. While these various alloy development efforts often had different goals for the end product alloy, some being developed with the intent of improving high temperature capability, some with improved corrosion resistance, and even some with improved forging/forming capabilities, perhaps the most common goal was simply tensile strength capability. In this case, tensile strength implies “useable” tensile strength, i.e., at an acceptable ductility level. Since strength and ductility vary inversely with each other, as is the case for virtually all hardenable metal systems, one usually has to make trade-offs between strength and ductility in order to obtain an alloy that is useful for engineering applications.
Standard (uniaxial) tensile properties are usually described by four properties determined in a routine tensile test: yield strength (YS), ultimate tensile strength (UTS, commonly referred to simply as “tensile strength”), % Elongation (% El) and % Reduction in Area (% RA). The first two values are usually reported in units such as ‘ksi’ (thousands of pounds per square inch) while the later two (both measures of ductility) are simply given in percentages.
Another tensile property often cited, particularly in reference to fastener applications, is “double shear” strength, also reported in ksi. For this property, ductility is not determined, nor is a yield strength. In general, double shear strength of titanium alloys are approximately 60% of the uniaxial tensile strengths, as long as uniaxial ductility is sufficient.
When attempting to make comparisons of tensile properties from different alloys heat treated to a range of tensile strength/ductility combinations, it is convenient to first analyze the data by regression analysis. The strength/ductility relationship can usually be described by a straight-line x-y plot wherein the ductility (expressed as either % El or % RA) is the dependent variable and the strength (usually UTS) is the independent variable. Such a line can be described the simple equation:% RA=b−m(UTS);  Eqn 1where m=the slope of the straight line and b is the intercept at zero strength. [Note: When determining such an equation by regression analysis, a parameter referred to as “r-squared ” is also calculated, it varies between zero and one—with a value of one indicating a perfect fit with the straight line equation and a value of zero indicating no fit].
Once such an equation is established, it can be used, for example, to compare ‘calculated’ ductilities at a constant strength level, even if there is no specific data at that strength level. This methodology has been used throughout this development effort in order to rank and compare alloys.
It should also be noted that when conducting an alloy development project, it is important to recognize that tensile strength/ductility relationships are significantly affected by the amount of hot-work that can be imparted to the metal during conversion from melted ingot to wrought mill product (such as bar). This is due to the fact that macrostructure refinement occurs during ingot conversion to mill product and the greater the macrostructure refinement the better the strength/ductility relationships. It is thus well understood by those skilled in the art that tensile strength/ductility relationships of small lab heats are significantly below those obtained from full sized production heats due to the rather limited amount of macrostructure refinement imparted to the small laboratory size heats compared to full-sized production heats. Since it is a practical impossibility to make full-size heats and convert them to mill product in order to obtain tensile property comparisons, the accepted practice is to produce smaller lab-sized heats of both the experimental alloy formulations and an existing commercial alloy formulation and compare results on a one-to-one basis. The key is to choose a commercial alloy with exceptional properties. In the development program resulting in this invention, the commercial alloy designated as “Ti-17” (Ti-5A1-2Sn-2Zr-4Cr-4Mo) was chosen as the baseline commercial alloy against which the experimental alloys would be compared. This alloy was chosen because of the exceptional strength/ductility properties demonstrated by this alloy in bar form.
TABLE 1Tensile and Shear Strength Data from acommercial high strength titanium alloy (Ti-17) processed to bar*AgeDoubleAvg DoubleAlloy Chemistry(Deg F. /UTSDoubleShear as %Shear a % of(wt %)HRS)YS (ksi(ksi)% EI% RAShear (ksi)of UTSUTSTi-17 (Ti-5Al-2Sn-1100/8182183124411462%2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn-″183184143911864%2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn-″189190113611359%2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn-″190192134111158%2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn-1050/819720093411558%59.8%2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn-″19820193011658%2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn-″205209822N/AN/A2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn-″205209828N/AN/A2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn- 950/12211216925N/AN/A2Zr-4Cr-4Mo)Ti-17 (Ti-5Al-2Sn-″212217929N/AN/A2Zr-4Cr-4Mo)Regression Analysis:% RA = 134.5 − 0.5080 (UTS)r − sq = 0.79% RA @ 195 UTS = 35.4% RA @ 215 UTS = 25.3% EL = 38.76 − 0.1427 (UTS)r − sq = 0.69% EL @ 195 UTS = 10.9% EL @ 215 UTS = 8.1*Material solution treated at 1480° F. for 10 min followed by fan air cool
Table 1 provides tensile and double shear property data for Ti-17 0.375 inch diameter bar product produced from a nominal 10,000 lb. full-sized commercial heat. The combinations of tensile strength, shear strength and ductility exhibited in this Table are clearly exceptional for any titanium alloy. Note also that the double shear strength values average very close to the 60% of UTS value cited earlier.